The present invention relates to a technique effectively applied to a semiconductor integrated circuit (RF module) for modulation which is used in a transmission system of a radio communication system employing frequency hopping for performing frequency modulation by controlling an LC oscillation type VCO (Voltage Controlled Oscillator) on the basis of transmission data while generating a carrier frequency signal by the LC oscillation type VCO and performing data transmission while hopping a carrier frequency. More particularly, the invention relates to a temperature compensating technique for an oscillation frequency of a VCO in a modulating operation performed in an open PLL loop which generates a carrier frequency.
In data communications in which signals of various communication systems are transmitted in association with development in radio communication, there is the possibility that accurate data transmission cannot be guaranteed due to interference of signals or fading. There is consequently a radio communication system preventing interference of signals in neighboring frequency bands by varying carrier frequency signals. For example, in the standard called Bluetooth defining specifications regarding data transmission by radio communication among peripheral devices such as a personal computer and a printer, as a transmission system, as shown in FIG. 13, spread spectrum realized by frequency hopping of every MHz in the frequency band of 2.4 GHz to 2.48 GHz (hereinbelow, called a 2.4 GHz band) is employed to prevent interference of signals in neighboring frequency bands. The Bluetooth standard employs a frequency modulating method of transmitting data while modulating a carrier frequency signal in the 2.4 GHz band with xc2x1160 kHz.
In the case of performing such frequency modulation, a method of controlling the frequency by directly controlling a VCO circuit in accordance with data to be transmitted may be employed. As VCO circuits, a circuit of a type controlling an oscillation frequency by changing a current by a control voltage and a circuit called an LC oscillation type VCO which controls an oscillation frequency by changing a capacitance value of variable capacity by a control voltage are known.
For example, in the case of modulating a frequency in accordance with transmission data by frequency hopping, there are not only the frequency modulation according to data but also hopping control of a carrier frequency itself, so that two control systems are necessary. The inventors herein have therefore examined the following system in order to develop an LSI (large scale semiconductor integrated circuit) for radio communication employing the radio communication system. According to the examined system (hereinbelow, called an open loop system), an LC oscillator type VCO using a varactor diode as shown in FIG. 2 is used. The frequency is stabilized by a PLL loop while switching the frequency by changing a control voltage Vcnt1 as one of control voltages in accordance with the carrier frequency hopped. At the time of performing modulation, while holding the control of voltage Vcnt1 in a loop filter, the PLL loop is opened, and the oscillation frequency of the VCO is changed with a control voltage Vcnt2 according to transmission data, thereby modulating the carrier frequency signal.
In the case of performing modulation by directly controlling a VCO with transmission data by using the LC oscillation type VCO, in a closed loop system for performing modulation without opening the PLL loop, when a clock which is stable irrespective of changes in temperature is used as a reference clock of which phase is compared with the phase of a feedback oscillation signal from the VCO, even if the capacitance value of the LC oscillation type VCO changes due to a temperature change, the VCO can be oscillated at a desired frequency. However, it became apparent that, in the case of employing the open loop system, a modulation frequency of the VCO largely fluctuates according to a temperature change, and a problem such that a margin is severely narrowed in a permissible range occurs. The open loop system has an advantage that the circuit configuration is simpler and a circuit occupation area can be reduced as compared with the closed loop system.
On the other hand, for example, in the Bluetooth standard, in the case of transmitting a signal obtained by modulating a carrier frequency signal in the 2.4 GHz band with xc2x1160 kHz, it is sufficient to modulate the signal within the range from xc2x1145 to 175 kHz. That is, the margin of xc2x115 kHz is permitted. As long as the modulation is within the margin, the frequency may fluctuate more or less due to temperature change. However, when the margin of the frequency is narrowed due to a temperature change, the strain is shifted to designing of other circuits, and it becomes very difficult to design a transmitting circuit having characteristics satisfying the standard.
For example, in the case of performing modulation with the same amplitude in accordance with transmission data, in other words, in the case of controlling an oscillation frequency with a fixed amplitude of a control voltage Vcnt2 irrespective of the carrier frequency in the VCO in FIG. 2, when the frequency is switched by the other control voltage Vcnt1 for frequency hopping, not only the capacitance of varactor diodes Dv11 and Dv12 but also a composite capacitance of the varactor diodes Dv11 and Dv12 and other varactor diodes Dv21 and Dv22 change and the frequency changes. It makes the modulation gain of the VCO change and the modulation frequency may shift according to the carrier frequency. Therefore, it is desirable to suppress fluctuations in frequency in association with the temperature change as much as possible.
It became apparent that the frequency fluctuates due to the temperature change for the following reason. In the LC oscillator type VCO as shown in FIG. 2, when a composite capacitance of capacitors C11 and C12 and the varactor diodes Dv11 and Dv12 is C1 and a composite capacitance of capacitors C21 and C22 and the varactor diodes Dv21 and Dv22 is C2, characteristics with respect to voltage of the composite capacitances C1 and C2 are as shown in FIG. 4A. It is understood from FIG. 4A that the capacitance value of each of the composite capacitances C1 and C2 increases as the temperature rises.
In this case, an oscillation frequency focs of the VCO is expressed as fosc=1/{2xcfx80{square root over ( )}(C1+C2)}. From the expression, it is understood that when the capacitance values of the composite capacitances C1 and C2 increase as the temperature rises, the oscillation frequency fosc of the VCO decreases. The frequency characteristics with respect to the control voltage Vcnt1 of the VCO were examined and it was found that the frequency characteristics are as shown in FIG. 4B. Specifically, it is understood from FIG. 4B that as the temperature rises, a change in frequency increases, and the gradient of a tangent line of a curve between the upper and lower limits in the control voltage range, that is, the variation in the voltage-frequency control sensitivity of the VCO also increases. In a communication system of the Bluetooth standard of hopping the frequency in the range from 2.402 GHz to 2.480 GHz, the oscillation frequency of the VCO is 2.480 GHz at the maximum and is 2.402 GHz at the minimum, and the difference between the maximum and minimum frequencies is very large. Consequently, it is considered that the variation in the voltage-frequency control sensitivity of the VCO is also large.
An object of the invention is, therefore, to reduce fluctuations in modulation frequency of a VCO in association with a temperature change in a semiconductor integrated circuit for modulation for use in a frequency hopping type radio communication system which modulates a frequency by controlling a VCO circuit in accordance with transmission data in an open loop method by using an LC oscillation type VCO circuit.
Another object of the invention is to provide a semiconductor integrated circuit for modulation which can serve as a component of a radio communication system capable of performing accurate data transmission with little crosstalk.
The above and other objects and novel features of the present invention will become apparent from the description of the specification and the appended drawings.
The outline of representative ones of inventions disclosed in the application will be described as follows.
In a semiconductor integrated circuit for modulation which performs frequency modulation by controlling an LC oscillation type VCO on the basis of transmission data while generating a carrier frequency signal by the LC oscillation type VCO and performs data transmission while hopping a carrier frequency, a temperature characteristic correcting circuit for making a modulation-side control input voltage (Vcnt2) for controlling the VCO have a negative temperature characteristic by giving a negative temperature characteristic to a reference current value of a circuit (for example, a DA converter) for generating a control voltage of the VCO is provided.
With the means, even when the VCO has a positive temperature characteristic, a voltage controlling the VCO is allowed to have a negative temperature characteristic. Consequently, a temperature-compensated oscillation frequency having no temperature dependency is obtained. Thus, the frequency margin can be increased and the circuit designing is facilitated. In the case of applying the semiconductor integrated circuit for modulating having such a configuration to a radio communication system of a frequency hopping method, the precision of the oscillation frequency obtained is increased. Therefore, a radio communication system capable of performing accurate data transmission with little interference can be realized.
The invention is particularly effective to a semiconductor integrated circuit including: a voltage controlled oscillator; a phase comparator for comparing a phase of an oscillation output of the voltage controlled oscillator with a phase of a reference clock signal; and a control voltage generating circuit for generating a voltage which eliminates a phase difference detected by the phase comparator and applying the voltage as a first control voltage to the voltage controlled oscillator. By a phase locked loop constructed by the voltage controlled oscillator, the phase comparator, and the control voltage generating circuit, a carrier frequency is given. A second control voltage for modulation is supplied to the voltage controlled oscillator from a path different from the phase locked loop. When the modulating operation is performed with the second control voltage, the phase locked loop is opened.
When a circuit for generating the second control voltage is constructed by a digital filter for sampling a digital transmission data signal and performing computation and a DA converter for DA converting an output of the digital filter, the temperature characteristic correcting circuit may correct so that a reference current of the DA converter has a negative temperature characteristic. When the circuit has a trimming circuit for adjusting the intensity of the reference current of the DA converter and the trimming circuit is constructed by a second DA converter for DA converting trimming information and supplying, as a reference current, a current according to the trimming information to the DA converter, the temperature characteristic correcting circuit makes correction so that the reference current of the second DA converter for trimming has a negative temperature characteristic.
Further, the temperature characteristic correcting circuit can be constructed by a plurality of first current sources for passing a current having no temperature dependency, a plurality of second current sources for passing a current having a positive temperature characteristic, and selecting means for selecting one or more current sources from the plurality of first current sources and from the plurality of second current sources, and a composite current of the selected current sources can be output as the reference current.